Thursday, July 16, 2009

Videos of a series of lectures from 1964 by Physics Nobel Prize winner Richard Feynman has been placed on the web at Microsoft."This law has been called the greatest generalization achieved by the human mind. And you can get already from my introduction, that I'm interested not so much in the human mind as in the marvel of nature, who can obey such an elegant and simple law as this law of gravitation. So our main concentration will not be on how clever we are to have found it all out, but on how clever she is to pay attention to it."

The lithium problem arises from the significant discrepancy between the primordial 7Li abundance as predicted by BBN theory and the WMAP baryon density, and the pre-Galactic lithium abundance inferred from observations of metal-poor (Population II) stars. This problem has loomed for the past decade, with a persistent discrepancy of a factor of 2--3 in 7Li/H.

Measurements of the primordial baryon-to-photon ratioη from the cosmic microwave background fromWMAP [1], coupled with precise measurements of theneutron half-life [2], have made big bang nucleosynthesis(BBN) an essentially parameter-free theory [2, 3, 4]. Inthis paradigm excellent agreement has been obtained betweenpredicted and observed abundances of deuteriumand 4He (see, e.g. the Particle Data Group review [2] andreferences therin). However there is some disagreementfor 7Li, the only other element for which the abundancehas been measured to an accuracy at which fruitful comparisonwith theory can be made. While the “lithiumproblem” has been known for some time, it has been exacerbatedby recent measurements of the 3He(α, γ)7Bereaction [5]. Standard BBN theory with η providedby WMAP 5 overproduces 7Li by a factor of 2.4 – 4.3(around 4 – 5σ) [4].

There are at least four unexplained anomalies connected with astrometric data. Perhaps the most disturbing is the fact that when a spacecraft on a flyby trajectory approaches the Earth within 2000 km or less, it often experiences a change in total orbital energy per unit mass. Next, a secular change in the astronomical unitAU is definitely a concern. It is increasing by about 15 cm yr$^{-1}$. The other two anomalies are perhaps less disturbing because of known sources of nongravitational acceleration. The first is an apparent slowing of the two Pioneer spacecraft as they exit the solar system in opposite directions. Some astronomers and physicists are convinced this effect is of concern, but many others are convinced it is produced by a nearly identical thermal emission from both spacecraft, in a direction away from the Sun, thereby producing acceleration toward the Sun. The fourth anomaly is a measured increase in the eccentricity of the Moon's orbit. Here again, an increase is expected from tidal friction in both the Earth and Moon. However, there is a reported unexplained increase that is significant at the three-sigma level. It is prudent to suspect that all four anomalies have mundane explanations, or that one or more anomalies are a result of systematic error. Yet they might eventually be explained by new physics. For example, a slightly modified theory of gravitation is not ruled out, perhaps analogous to Einstein's 1916 explanation for the excess precession of Mercury's perihelion.